Though relatively unknown and uncharacterized, the orphan GPCR LPHN3 has already been correlated in genome-wide linkage and association studies to both addictive phenotypes and Attention Deficit Hyperactivity Disorder (ADHD) independently. Vulnerability to addiction and ADHD are frequently comorbid, suggesting a common etiology. Further, initial data indicate that LPHN3 genotype may be a pharmacogenetic marker predicting response to stimulant medication. Unfortunately, as no disease-associated functional polymorphisms have been identified in LPHN3, its mechanism of action is completely unknown, and further characterization of its function is essential as there are far reaching implications for such an important gene. The central hypothesis to be tested in this proposal is that Lphn3 signalling affects neurotransmitter release and concentrations of monoamines in the brain that may affect neural cell differentiation, development, and function. These alterations ultimately play out in an effect on basic behavior that may influence phenotypes such as ADHD and vulnerability to addiction. We propose to utilize the novel Lphn3 null mutant mouse to investigate gene function. Preliminary data indicate that the Lphn3 null mouse is hyperactive, has altered DA and 5-HT levels in the brain, and demonstrates altered expression levels of genes involved in neuronal differentiation and development as well as other ADHD candidate genes involving the dopaminergic and serotonergic pathways including dopamine and serotonin receptors. These scant clues have been used by us to construct our research plan. We will begin with defining the Lphn3 developmental expression pattern in the brain and look at co-expression with other well-characterized neuronal markers. We will also utilize TaqMan Gene Expression Assays to look for changes in expression of genes known to play a role in neuronal differentiation, development, survival, and function as a function of Lphn3 genotype. Assays will be preformed across developmental time points and in specific brain regions. Next we will isolate primary neurons from WT and mutant mice and evaluate their viability, growth and function based on Lphn3 genotype. Finally, we have designed a battery of behavioral assays to begin to evaluate the basic behavioral deficits in the Lphn3 mice. This battery includes preliminary assessment of: activity, motor function, anxiety, depression, motivation, attention, impulsivity, learning and memory. Data derived from this project will be a first step toward characterizing the basic functions of Lphn3, which could enable better understanding, diagnosis, and treatment of ADHD and addiction.